1. Field of the Invention
The present invention relates to a positive electrode active material including a compound oxide mainly composed of lithium and nickel, a non-aqueous electrolyte secondary battery containing the positive electrode active material, and in particular, to the technology to improve the high temperature property of the non-aqueous electrolyte secondary battery containing the positive electrode active material.
2. Description of the Related Art
Many kinds of portable electronic equipment such as camcorders, cell phones, and laptop computers have been developed in recent years and the demand has been rapidly increasing. As the reduction in size and in weight of the electronic equipment progresses, intensive research and development for improving the energy density of a battery, i.e. the portable power supply, in particular, a second battery have been done. In particular, the demand for a secondary lithium ion battery has been increasing in which doping and undoping of lithium ions are employed. The reason is that the energy density of the secondary lithium ion battery is larger than that of known aqueous electrolyte secondary batteries such as a lead battery, a nickel-cadmium battery, and a nickel-hydrogen battery. In addition, the improvement in the environmental resistance of the secondary lithium ion battery is also expected in order to expand the field of application.
Examples of the positive electrode active material practically used for the lithium ion battery include lithium cobalt oxide and lithium nickel oxide both of which have a layered rock-salt structure, and lithium manganese oxide that has a spinel structure. Each oxide has advantages and disadvantages. Lithium cobalt oxide is widely used currently because the oxide particularly has the superior balance of, for example, capacity, cost, and thermal stability. On the other hand, lithium manganese oxide has a small capacity, and is slightly inferior in high temperature storage property. Lithium nickel oxide is slightly inferior in the stability of the crystal structure, and has some disadvantages in the cycle durability and in the environmental resistance, in particular, in the high temperature property. However, since lithium manganese oxide and lithium nickel oxide have advantages in the material price and in the stability of the supply compared with lithium cobalt oxide, lithium manganese oxide and lithium nickel oxide are more of a promising material and have been studied.
In order to improve the cyclic durability of the lithium nickel oxide, disclosed methods include a method in which the nickel is partly replaced with another element (see, for example, Japanese Unexamined Patent Application Publication Nos. H08-37007 and 2001-35492), a method in which, for example, a certain metal salt is added (see, for example, Japanese Unexamined Patent Application Publication No. H07-192721), and a method in which a specific binder is used in the positive electrode mixture (see, for example, Japanese Unexamined Patent Application Publication No. H10-302768). However, according to the research by the present inventors, in the methods disclosed in the above patent documents, the environmental resistance, in particular, the high temperature property was not sufficiently improved.
Furthermore, in order to improve the characteristics of the positive electrode active material including lithium nickel oxide, a method in which the surfaces of the positive electrode active material are coated with a conductive material or other layered oxide is disclosed (see, for example, Japanese Unexamined Patent Application Publication Nos. H07-235292, H11-67209, and 2000-149950). Unfortunately, in the methods disclosed in the above patent documents, the coating on the positive electrode active material was not uniform, and a large amount of the coating must be required in order to efficiently improve the characteristics. Since these phenomena decrease the capacity of the battery, it is difficult to apply the above methods to a battery having a high capacity.
In addition, a method in which a metal or a metal oxide both of which barely decompose a non-aqueous electrolytic solution is dispersed and is held on the surfaces of the positive electrode active material is also disclosed (see, for example, Japanese Unexamined Patent Application Publication No. H08-102332). However, the metal or the metal oxide disclosed in the above patent document had a significantly low conductivity of lithium ions. Consequently, the metal or the metal oxide on the surfaces of the positive electrode active material prevented lithium ions from doping and undoping in the positive electrode active material. The non-aqueous electrolyte secondary battery using this positive electrode active material had an insufficient performance in the practical use. In addition, the disclosed amount of the metal or the metal oxide used for the dispersion did not have a sufficient effect.
Furthermore, a method in which a surface layer containing titanium (Ti) is formed on the surfaces of the positive electrode active material is disclosed (see, for example, Japanese Unexamined Patent Application Publication Nos. 2002-63901 and 2001-256979). However, according to the methods, the presence of lithium on the surface layer is not considered and the amount of the coating disclosed in Japanese Unexamined Patent Application Publication No. 2002-63901 could not sufficiently improve the characteristic in an operation at a high temperature.
In addition, disclosed positive electrode active materials include a positive electrode active material produced by coating particles of lithium compound oxide with a layer composed of an alkali metal compound or a metal oxide (see, for example, Japanese Unexamined Patent Application Publication No. 2001-313034), a positive electrode active material produced by forming a layer composed of a compound oxide of lithium and a transition metal on the surfaces of particles of lithium compound oxide (see, for example, Japanese Unexamined Patent Application Publication No. H08-162114), and a non-aqueous electrolyte secondary battery including a positive electrode active material having lithium cobalt oxides and a lithium oxide compound such as LiTi2O4 (see, for example, Japanese Patent No. 2797390). For example, according to the art disclosed in Japanese Unexamined Patent Application Publication No. 2001-313034, the coating layer is composed of a mixture including an alkali metal compound and a metal oxide. Therefore, this method is different from the art using a compound oxide. According to the art disclosed in Japanese Unexamined Patent Application Publication No. H08-162114, the combination of elements in the particles and the coating layer is not specifically considered. According to the art disclosed in Japanese Patent No. 2797390, the lithium cobalt oxides and a lithium compound are just mixed, and the art does not include a concept in which a coating layer composed of a lithium compound is formed.
As described above, in some known art, a coating layer is formed on the positive electrode active material. However, significantly improving the high temperature property of the lithium nickel oxide, which is a drawback of the lithium nickel oxide, without decreasing the conductivity of lithium ions due to the formation of the coating layer is very difficult and has not been achieved yet.